Binder-free molecular sieve zeolite granules which contain zeolites of the type lithium zeolite A and lithium zeolite X

ABSTRACT

The present invention relates to binder-free molecular sieve zeolite granules of lithium zeolite A and lithium zeolite X, a process for preparing these molecular sieve zeolite granules and their use for preparing nitrogen or oxygen from air by pressure swing adsorption.

The present invention relates to binder-free molecular sieve zeolitegranules which contain zeolites of the type lithium zeolite A andlithium zeolite X, a process for preparing these molecular sieve zeolitegranules and their use for preparing nitrogen or oxygen from air bypressure swing adsorption.

The production of oxygen from air at ambient temperatures (e.g. -30° C.to +50° C.) is generally performed on an industrial scale usingmolecular sieve zeolites. Here, the preferential adsorption of nitrogenas compared with oxygen is used, i.e. oxygen and argon from air arecollected as product at the discharge point after the air has passedthrough a zeolite bed. Desorption of the adsorbed nitrogen can beperformed, for example, by reducing the pressure in the bed. In thiscase, the process is called vacuum swing adsorption (VSA) in contrast tothe also known pressure swing adsorption process (PSA), wherein thenitrogen is desorbed at approximately ambient pressure. A continuous VSAprocess is characterised by the following process steps:

a) passage of air through a zeolite bed (e.g. at ambient pressure) andwithdrawal of O₂ -rich gas from the discharge zone;

b) reduction of pressure in the bed to, for example, about 100 to 400mbar, using a vacuum pump, in counterflow to the flow of air;

c) filling the bed with O₂ -rich gas in counterflow to the stream of airor with air in co-current flow with the stream of air to the adsorptionpressure or approximately to the adsorption pressure.

The objective of the various processes is always a high product rate,with reference to the amount of zeolite combination used, and a high O₂yield (ratio of the amount of O₂ in the product to the amount of O₂ inthe quantity of air used). A high O₂ yield includes a low energy demand(with reference to the amount of O₂ produced) for the vacuum pump or aircompressor.

As a result of the three steps mentioned above, three zeolites aregenerally used, i.e. three adsorbers, which are operated in a cycle.

The economic viability of these types of adsorption units is affected bythe investment such as, for instance, the amount of adsorption agent andthe size of the vacuum pumps and in particular by the operating costssuch as, for example, power consumption by the vacuum pump and/or theair compressor. Therefore, zeolites have been developed with which it ispossible to achieve high nitrogen adsorptions in the range between theadsorption pressure and minimal desorption pressure, so that the amountof zeolite used can be kept at a low level or even reduced. As describedin EP-A 374 631, Ca zeolites A have been used for this purpose. Furtherdevelopments in this area are directed at increasing the selectivity fornitrogen as compared to oxygen.

Higher selectivity is achieved by using lithium zeolite X (EP-A 297542). A higher separation factor (N₂ loading to O₂ loading) and a higherN₂ loading are obtained than with Na zeolite X.

U.S. Pat. No. 5,174,979 describes granules bonded with clay minerals,the zeolite fraction consisting of Li zeolite A or Li zeolite X, whereinthe Li₂ O/Al₂ O₃ ratio in the Li zeolite A granules is between 10 and70% and the Li₂ O/Al₂ O₃ ratio in the Li zeolite X granules is between50 and 95% and the remaining cations are calcium or strontium ions. Atan air pressure of 1 bar (abs.), pure lithium zeolite A granulesdemonstrate an N₂ adsorption of only about 0.35 mmol/g. equ.; and the N₂adsorption on Li zeolite X granules at 0.8 bar (abs.) is about 1.1mmol/g. equ.

Granules consisting of lithium zeolite A can therefore be improved byintroducing additional calcium or strontium ions in an exchange process.

In EP-A 0 548 755, it is shown that in the case of lithium zeolite X,the N₂ adsorption and N₂ /O₂ selectivity does not decreasesubstantially, in comparison with a completely exchanged lithium zeoliteX, by introducing calcium and strontium ions as long as the amount ofNa₂ O in the zeolite lattice remains small. According to FIG. 5 in thisdocument, a zeolite X completely exchanged with lithium has only about34% higher "nitrogen working capacity" than a zeolite X completelyexchanged with calcium ions. According to FIG. 7 in this document, theN₂ /O₂ selectivity of Ca zeolite is in fact about 10% (in relativeterms) better than lithium zeolite X.

In EP-A 297 542, to prepare lithium zeolite X granules, Na zeolite Xpowder is bonded with clay, then calcined, moistened again, exchangedwith a LiCl solution, washed with LiOH and finally activated with a hotstream of gas, i.e. rendered anhydrous.

DE-A 1 203 238 discloses granules which consist of Na zeolite A, inwhich the SiO₂ binder is converted into zeolite A in an after treatmentstep, a so-called aluminising process. The components called binders areinactive constituents of the granules which bind the zeolite powder toproduce granules (beads or sections of extruded strands). The N₂ and O₂loading on the inactive binder is minimal.

DE-A 3 401 485 describes the preparation of SiO₂ -bonded zeolite A andzeolite X granules.

According to EP-A 0 170 026, in particular example 2, fracture resistantgranules are disclosed, these consisting of Ca zeolite A and a SiO₂binder and being advantageously used in accordance with EP-A 0 374 631for the oxygen enrichment of air.

DE-A 1 203 238, in particular example 7, discloses granules whichconsist of Na zeolite X in which the SiO₂ binder has been converted intozeolite A in an aftertreatment step. The disadvantage of zeolitegranules which consist of Na zeolite X and/or Na zeolite A and a SiO₂binder is the low fracture strength of these granules, wherein this isindependent of the shape of the granules (beads or rods).

Treating the SiO₂ binder in zeolite A or zeolite X granules withsolutions of salts of alkaline earth metal cations increases thefracture strength of the granules. Converting the inactive SiO₂ binderinto active Na zeolite A should also increase the adsorption capacity ofthe entire granular material. Increasing the fracture strength ofgranules made of sodium zeolite A or sodium zeolite X and a SiO₂ binderby treatment with a solution of a lithium salt is not possible.

DD 0 154 690 discloses a process for separating oxygen from gases,wherein binder-free molecular sieve zeolite granules of the type NaLizeolite A are used (see page 4, Table 1 and page 5, example 1). In Jzv.Akad. Nauk SSSR, Ser. Khim. 1966 (10), 1869 (CA66:70743f), the ionexchange of a sodium zeolite to give Li zeolite A and the X-raystructure of and NMR data for corresponding pellets are also described.

The object was to provide binder-free, but fracture-resistant, Lizeolite granules which can be prepared in a technically simple manner,which can be used for the separation of air in a pressure swingadsorption process and which ensure a high yield for oxygen and highproduct capacity.

The invention provides abrasion-resistant, fracture-resistant,binder-free molecular sieve zeolite granules which are characterised inthat the granules contain finely distributed zeolites of the types Lizeolite A and Li zeolite X.

The granules preferably contain 10 to 90 wt. % of Li zeolite A,preferably 15 to 85 wt. % of Li zeolite A and at the same time 10 to 90wt. % of Li zeolite X, preferably 15 to 85 wt. % of Li zeolite X.

In particular, the Li zeolite A preferably has a degree of Li exchangeof 60 to 100%, with reference to exchangeable cations and the Li zeoliteX preferably has a degree of Li exchange of 60 to 100%, with referenceto exchangeable cations.

The Li zeolite A preferably contains up to 20 mol-% of divalent cations,and the Li zeolite X preferably contains up to 20 mol-% of divalentcations.

The invention also provides a process for preparing molecular sievezeolite granules according to the invention which is characterised inthat powdered zeolites of the type Na zeolite A and Na zeolite X aremixed with silica sol or other suitable SiO₂ -containing binders andmoulded to give SiO₂ -bonded granules, the granules obtained in this wayare aluminised, wherein the SiO₂ binder is converted into Na zeolite A,a Li exchange is performed with the granules treated in this way,wherein 60 to 100% of the exchangeable cations in the zeolite areexchanged and then the exchanged granules are subjected to thermaltreatment at temperatures of 300 to 650° C. in order to remove water(so-called activation).

An exchange with divalent cations from the group magnesium, calcium,barium, strontium, zinc, iron, cobalt or nickel, is preferably alsoperformed, before or after Li exchange.

Preferred SiO₂ -containing binders are, for instance, waterglasses,silica sols, silica gels, aerosils or silica fillers. Silica sols areparticularly suitable.

So-called aluminisation in the process according to the invention isperformed as follows:

The granules produced by granulation, in the moist state, are placed incontact with an aqueous sodium aluminate solution for several hours atan elevated temperature. The aluminate concentration is chosen to be ashigh as possible (preferably 0.5 to 2.0 mol of Al₂ O₃ per liter) inorder to keep the volume of treatment solution small. The amount ofaluminate solution is such that there are at least 0.5 moles of Al₂ O₃to 1 mole of SiO₂ binder. More than 0.5 moles of Al₂ O₃ does not causeany problems. The concentration of caustic in the aluminate solution(treatment solution) may vary between 1.5 and 10 moles of alkali metalhydroxide per liter.

Aluminisation is preferably performed (see also DE-A 1 203 238, example7) in such a way that, in a first step, the granules are treated withaluminate solution for 0.5 to 15 hours at a temperature of 25° C. to 60°C. Then, in a second step, treatment is continued for 2 to 6 hours attemperatures between 70 and 90° C. The precise treatment times dependmainly on the diameter of the granules. The smaller the diameter of thegranules, the shorter the treatment times (residence times). After thisprocedure, the amorphous SiO₂ binder is entirely converted into Nazeolite A (crystalline zeolite phase). After optionally then washing thegranules, these may be subjected directly to an ion exchange process.

Activation is preferably performed as follows:

Thermal treatment of the granules takes place at temperatures between300 and 650° C., this preferably being performed with dry gases such as,for instance, air or nitrogen.

Zeolite granules according to the invention may be used for theseparation of air in a pressure swing adsorption process.

The invention is explained in more detail by the following examples.

EXAMPLES

All granular samples had a particle size between 1 and 2.5 mm and wereactivated to a water content of less than 0.5 wt. % at the end of thepreparation process, using a stream of nitrogen heated to 600° C.Activation was performed as follows: The hot stream of nitrogen waspassed through a bed consisting of the zeolite granules at a spacevelocity of 10 Nm³ N₂ /h/10 dm³ of zeolite, until the dischargetemperature reached about 90% of the value of the inlet temperature. Theactivation time was extended by 20% beyond that point. The hot granuleswere then packaged. The activation temperatures normally used are 300 to650° C. The space velocity can be varied over wide limits without anyappreciable change in zeolite quality. Atmospheric air may also be used,instead of nitrogen. Dry gas generally reduces the residual watercontent. In addition to fixed beds, rotating tubes may also be used foractivation, the granules being treated continuously in these while thetube is flushed out with a hot stream of gas.

Sample A (Ca zeolite X with SiO₂ binder)

Zeolite X granules were prepared in the same way as described in EP-A 0170 026, example 2. The starting material was granules withapproximately 82% of Na zeolite X and 18% of SiO₂ as binder, wherein Caexchange was performed by treating with calcium chloride solution. TheCaO/Al₂ O₃ ratio was about 0.75. The SiO₂ /Al₂ O₃ ratio in the zeolite Xpowder used was about 2.3. The bulk density of the active granules was590 g/l.

Sample B (Ca zeolite X without SiO₂ binder)

Zeolite X granules were prepared in accordance with DE-A 20 16 838,example 2. The starting material was granules with approximately 82% ofNa zeolite X and 18% of SiO₂ as binder, wherein the SiO₂ fraction wasconverted into Na zeolite A by treating with aluininate and then a Caexchange was performed by treating with calcium chloride solution. TheCaO/Al₂ O₃ ratio in the granules was about 0.75. The SiO₂ /Al₂ O₃ ratioin the zeolite X powder used was about 2.3. The bulk density of theactive granules was 680 g/l.

Sample C (Li zeolite with SiO₂ binder)

Zeolite A granules were prepared in the same way as described in EP-A 0170 026, example 2. The starting material was granules withapproximately 82% of Na zeolite A and 18% of SiO₂ as binder, wherein Liexchange was performed by treating with lithium chloride solution. Here,60 liters of 1-molar lithium chloride solution per liter of granuleswere pumped through a bed of granules. The temperature was 85° C. Aftercompleting the ion exchange process, the granules were washed with waterwhich had been adjusted to a pH of 9 with LiOH. The degree of exchange(Li₂ O/Al₂ O₃) of the zeolite was 98% after ion exchange. The bulkdensity of the active granules was 600 g/l.

Sample D (mixture of Li zeolite X and Li zeolite A with SiO₂ binder)

Zeolite X granules were prepared in accordance with DE-A 20 16 838,example 4d. The starting material was granules consisting of a mixtureof approximately 65% of Na zeolite X and 17% of Na zeolite A and 18% ofSiO₂ as binder, i.e. zeolite proportions of 80% X and 20% A, wherein Liexchange was performed by treating with lithium chloride solution. Here,70 liters of 1-molar lithium chloride solution per liter of granuleswere pumped through a bed of granules. The temperature was 85° C. Aftercompleting the ion exchange process, the granules were washed with waterwhich had been adjusted to a pH of 9 with LiOH. The degree of exchange(Li₂ O/Al₂ O₃) of the zeolite was 98% after ion exchange. The SiO₂ /Al₂O₃ ratio in the zeolite X powder used was about 2.3. The bulk density ofthe active granules was 605 g/l.

Sample E (mixture of Li zeolite X and Li zeolite A without SiO₂ binder)

Zeolite X granules were prepared in accordance with DE-A 20 16 838,example 4d. The starting material was granules consisting of a mixtureof about 65% of Na zeolite X and 17% of Na zeolite A and 18% of SiO₂ asbinder, wherein the SiO₂ fraction was converted into Na zeolite A byaluminate treatment and then, after a wash process, Li exchange wasperformed by treating with lithium chloride solution. Here, 70 liters of1-molar lithium chloride solution per liter of granules were pumpedthrough a bed of granules. The temperature was 85° C. After completingthe ion exchange process, the granules were washed with water which hadbeen adjusted to a pH of 9 with LiOH. The degree of exchange (Li₂ O/Al₂O₃) of the zeolite was 98% after ion exchange. The SiO₂ /Al₂ O₃ ratio inthe zeolite X powder used was about 2.3. The bulk density of the activegranules was 660 g/l.

Example F (mixture of Li zeolite X and Li zeolite A without SiO₂ binder)

Zeolite X granules were prepared in accordance with DE-A 20 16 838,example 2. The starting material was granules consisting of a mixture ofabout 82% of Na zeolite X and 18% of SiO₂ as binder, wherein the SiO₂fraction was converted into Na zeolite A by aluminium treatment andthen, after a wash process, Li exchange was performed by treating withlithium chloride solution. Here, 70 liters of 1-molar lithium chloridesolution per liter of granules were pumped through a bed of granules.The temperature was 85° C. After completing the ion exchange process,the granules were washed with water which had been adjusted to a pH of 9with LiOH. The degree of exchange (Li₂ O/Al₂ O₃) of the zeolite was 98%after ion exchange. The SiO₂ /Al₂ O₃ ratio in the zeolite X powder usedwas about 2.3. The bulk density of the active granules was 650 g/l.

Sample G (Li/Ca zeolite X without SiO₂ binder)

Zeolite X granules were prepared in accordance with DE-A 20 16 838,example 2. The starting material was granules with approximately 82% ofNa zeolite X and 18% of SiO₂ as binder, wherein the SiO₂ fraction wasconverted into Na zeolite A by aluminate treatment and then Li exchangewas performed by treating with lithium chloride solution. Here, 60liters of 1-molar lithium chloride solution per liter of granules werepumped through a bed of granules. The temperature was 85° C. Aftercompleting the ion exchange process, the granules were washed with waterwhich had been adjusted to a pH of 9 with LiOH. The degree of exchange(Li₂ O/Al₂ O₃) of the zeolite was 98% after ion exchange.

Then a Ca exchange was performed by treating with a calcium chloridesolution in such a way that the CaO/Al₂ O₃ ratio was adjusted to 0.15.

The SiO₂ /Al₂ O₃ ratio in the zeolite X powder used was about 2.3. Thebulk density of the active granules was 680 g/l.

To assess the samples, the following properties of the activated sampleswere measured: fracture hardness, nitrogen and oxygen adsorption at 1bar (abs.) and 25° C. and also the degree of air separation, withreference to oxygen recovery, in a pressure swing adsorption process.

This pressure swing process is explained in more detail using FIG. 1:

The plant consists of the following parts:

G=air blower

V=oil-driven vacuum pump

R=product compressor

A/B/C=adsorbers consisting of metal columns with an internal diameter of56 mm and an internal length of 2000 mm; filled with zeolite granules;these columns are surrounded by a water jacket and maintained at aconstant temperature of +25° C.

11A-14C=electromagnetic valves

15=manually controlled valve

L11-L14=inlet pipes.

Description of Process

Time 0 to 60 seconds:

Dry air flows, via air blower G, through pipe L12 at an ambient pressureof 1.03 bar (abs.) into adsorber A filled with zeolite granules. O₂product gas leaves adsorber A via valve 14A and reaches product blower Rvia pipe L13. Product blower R controls the product stream via arestrictor, which is not shown.

At the same time, the pressure in adsorber C is reduced from atmosphericpressure to 200 mbar within 60 seconds, via valve 12C and pipe L11. Thefinal pressure is set using a restrictor, which is not shown, upstreamof the vacuum pump.

At the same time adsorber B is filled with product gas, the pressurerising from 200 mbar to atmospheric pressure within 60 seconds, usingvalve 13B and pipe L14 and manual valve 15.

All valves not mentioned are closed.

Time 60 to 120 seconds:

Air flows through adsorber B. Adsorber C is filled with product. Thepressure in adsorber A is reduced.

Time 120 to 180 seconds:

Air flows through adsorber C. Adsorber A is filled with product. Thepressure in adsorber B is reduced.

The amount of product downstream of compressor R at 90% and the amountof gas pumped out were determined.

The following characteristics can be calculated. The amount of O₂produced in the product per adsorption cycle of 60 seconds and the O₂yield (=amount of O₂ in the product with reference to the amount of O₂in the air used). This O₂ yield is an indirect measure of the energydemand for producing oxygen in pressure swing adsorption processes.

The following values were determined:

                  TABLE 1    ______________________________________                      Adsorption  Pressure swing                Fracture                      isotherms   adsorption trial                hard- N.sub.2 ad-                              O.sub.2 ad-                                      Amount                                            O.sub.2                ness  sorption                              sorption                                      of O.sub.2                                            yield                 kg!   Nl/kg!  Nl/kg!  Nl/kg!                                             %!    ______________________________________    Sample A (Ca zeolite X +                  2.6     14      4.8   0.7   46.2    A with SiO.sub.2 binder),    comparison    Sample B (Ca zeolite X +                  3.5     14.5    4.7   0.85  48    A, without SiO.sub.2 binder),    comparison    Sample C (Li zeolite X                  1.2     18      3.85  1.3   57    with SiO.sub.2 binder),    comparison    Sample D (Li zeolite 80%                  1.2     17.2    3.8   1.25  56    X + 20% A with SiO.sub.2    binder), comparison    Sample E (Li zeolite 65%                  3.1     20      4.35  1.4   58.25    X + 35% A without SiO.sub.2    binder), according to the    invention    Sample F (Li zeolite 80%                  3.1     22      4.7   1.55  59    X + 20% A without SiO.sub.2    binder), according to the    invention    Sample G (Li/Ca zeolite                  4.2     24      4.98  1.65  60.5    X + A without SiO.sub.2    binder), according to the    invention    ______________________________________

Comparing Samples A and B

The aluminisation step has a positive effect on the O₂ product rate inthe case of the Ca X+A zeolite granules and no appreciable effect on theO₂ yield.

Comparing Sample F with Sample C

The aluminisation step provides an improvement in O₂ yield and O₂product rate in the case of Li zeolite X.

Comparing Sample F with Sample D

For the same zeolite X to zeolite A ratio, the O₂ yield and O₂ productrate are clearly improved by the aluminisation step.

Comparing Sample G with Sample F

The incorporation of Ca ions in Li zeolite (X+A) has increased the O₂product rate and yield still more.

We claim:
 1. Molecular sieve zeolite granules, characterized in thatsaid granules consist essentially of a mixture of zeolites of Li zeoliteA and Li zeolite X, said granules containing 10 to 35 wt. % of Lizeolite A and 65 to 90 wt. % of Li zeolite X wherein the Li zeolite Awas obtained by the conversion of a silicate binder.
 2. Molecular sievezeolite granules according to claim 1, characterised in that the Lizeolite A has a degree of Li exchange of 60 to 100%, with reference toexchangeable cations, and the Li zeolite X has a degree of Li exchangeof 60 to 100% with reference to exchangeable cations.
 3. Molecular sievezeolite granules according to claim 2, characterised in that the Lizeolite A contains up to 20 mol-% of divalent cations and the Li zeoliteX contains up to 20 mol-% of divalent cations.
 4. Molecular sievezeolite granules according to claim 3, characterised in that thedivalent cations are selected from the group consisting of magnesium,calcium, barium, strontium, zinc, iron, cobalt or nickel.
 5. A processfor preparing molecular sieve zeolite granules according to claim 1,characterised in that powdered Na zeolite A and Na zeolite X are mixedwith silica sol or other suitable SiO₂ -containing binders and molded togive SiO₂ -bonded granules, the SiO₂ -bonded granules are aluminised,wherein the SiO₂ binder is converted into Na zeolite A to produce themolecular sieve granules containing 10 to 35 wt. % of zeolite A and 65to 90 wt. % of zeolite X, a Li exchange is performed with the molecularsieve granules wherein 60 to 100% of the exchangeable cations in the Nazeolite A and Na zeolite X are exchanged and then the exchanged granulesare activated at temperatures of 300 to 650° C.
 6. Process according toclaim 5, further comprising exchanging said molecular sieve granuleswith divalent cations before or after Li exchange.